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Javier
sage
Reged: 05/03/09
Posts: 431
Loc: New Jersey
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Just how accurate are the settings circles that come with EQ mounts for six inch reflectors such as the Celestron Omni XLT's? I've used them in the past and it seems that once I set everything up and use the coordinates I'm usually off when I aim at an object. I'm not expecting 100% accuracy, but I figured they would get me close to an object once they are properly setup.
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Jimmy2K63
Pooh-Bah
Reged: 04/26/09
Posts: 1176
Loc: Kentucky
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Setting circles are only accurate if you are polar aligned and your co-ordinates are current for the year (or fairly close). You need to set them properly as well - pick a star and dial in the declination axis and fix it at that point, whether north or south of the celestial equator. For the RA (polar shaft), this will move from minute to minute and hour to hour so you may have to make some adjustments.
Depending on the diameter of the circles, they should be reasonably accurate, at least to within 1 degree.
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http://astronomyguy63.blogspot.com/
LXD75 SN6-UHTC
Cave Astrola 10" f/5
Garrett 15x70/FarSight
Canon XS (1000D)
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gnowellsct
professor emeritus
Reged: 06/24/09
Posts: 729
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I'll say what Jimm2k63 said, but differently. Analog setting circles are not "set it and forget it." First off, on an inexpensive mount you are rarely going to be really well polar aligned. So that will introduce error right off the bat. Second, there are other sources of pointing error, flexure and such, that introduce quite a bit of pointing error on their own.
So the best thing to do is get in the habit of continually adjusting your setting circles on well known objects. Point the scope to Vega, set to Vega's coordinates, and you should be OK finding M57, and M56, and m27, and M71, which are all "in the neighborhood."
There is another way to use setting circles like this. Now you don't set them to anything at all. You need a good sky atlas like the Sky Atlas 2000.
You point your scope at Vega and look at the sky chart. It says, you need to N degrees S in Dec and X hrs/minutes in RA to get to Vega. So you use the angular markings to go that far. If you do it this way it doesn't matter what the marked values are. If you need to move 4 degrees south it doesn't matter if it says 26 degrees (you move to 22) or 58 degrees (you move to 54), all you're interested in is moving the right number of degrees. The do the same in RA.
One technique is called the "dec sweep": you move to the correct position in RA and then just sweep in Dec until you fall on the object. Best done with a wide field eyepiece at low powers.
Setting circles actually work remarkably well once you get past the inhibition of thinking they have to be set and calibrated one way for the night.
Note that RA setting circles have both norhtern and southern hemisphere flavors if you try to use a southern hemisphere RA in the North it will do everything backwards. However, many RA circles actually have two sets of values on the markings, one for northern hemisphere, and the other, going in the opposite direction, for the southern hemisphere.
Greg N
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"Aperture will get you through times of no money better than money will get you through times of no aperture."
featuring selected astrojunk:
bunch o' widefield eyepieces
bunch o' narrowfield eyepieces
couple o' Barlows
couple o' scopes
couple o' mounts
couple o' tripods
and a pier 'n' stuff
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Javier
sage
Reged: 05/03/09
Posts: 431
Loc: New Jersey
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Thanks guys, I'm going to have to call Celestron and find out which RA numbers are for the North and which are for the South. The insttuction manual doesn't stat this.
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brianb11213
Carpal Tunnel
Reged: 02/25/09
Posts: 2075
Loc: 55.215N 6.554W
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Quote:
Depending on the diameter of the circles, they should be reasonably accurate, at least to within 1 degree.
They used to come with verniers allowing them to be read to one arc minute or better - and they were that accurate, if your mount was properly aligned, your coordinates were for the right epoch and, most critically, you have the correct sidereal time as it only takes a 4 second error to throw you out one arc minute in RA.
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Jimmy2K63
Pooh-Bah
Reged: 04/26/09
Posts: 1176
Loc: Kentucky
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Quote:
Thanks guys, I'm going to have to call Celestron and find out which RA numbers are for the North and which are for the South. The insttuction manual doesn't stat this.
If you are looking south with the scope, the hours of RA should be increasing as you move from west to east in a sweep. You can confirm this by looking at a good sky atlas and verify it for yourself at the scope.
--------------------
http://astronomyguy63.blogspot.com/
LXD75 SN6-UHTC
Cave Astrola 10" f/5
Garrett 15x70/FarSight
Canon XS (1000D)
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gnowellsct
professor emeritus
Reged: 06/24/09
Posts: 729
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As you turn the polar aligned mount East in the N hemisphere, the RA numbers should INCREASE. Turn towards rising objects = increasing number. The only exception is from 23 to 0 when you start all over again.
If the numbers decrease as the scope travels "more to the east" then it is a southern circle. It might have one set of numbers up top and one on the bottom, a dual set.
If you use it simply as an angle measure, as I suggested, it doesn't matter which way the numbers go.
Greg N
--------------------
"Aperture will get you through times of no money better than money will get you through times of no aperture."
featuring selected astrojunk:
bunch o' widefield eyepieces
bunch o' narrowfield eyepieces
couple o' Barlows
couple o' scopes
couple o' mounts
couple o' tripods
and a pier 'n' stuff
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